1. Field of the Invention
The present invention relates to a crack monitoring method and apparatus for monitoring the length of a crack generated inside a weld in high-temperature thick-walled piping of a plant while the plant continues to operate.
2. Related Background Art
In an electric power station or the like, a periodic inspection is performed of welds and the like in high-temperature thick-walled piping, for example, a main steam pipe (Thickness: Approx. 100 mm) or a high-temperature reheat steam pipe (Thickness: Approx. 30 to 40 mm) of a plant in a non-destructive test method such as an ultrasonic test (UT) method or a time of flight diffraction (TOFD) method to check for the presence or absence of a crack and its length.
If a crack is found during the periodic inspection, the probable life of the piping is calculated on the basis of the length of the crack and the operating conditions of the plant to make a repair plan while the piping is used as it is until the plant is repaired since it is very hard to replace the cracked piping immediately.
As set forth in the above, the probable life of the cracked piping is calculated on the basis of the size of the crack and the operating conditions of the plant, and therefore while the plant can continue to be used without any problem within this probable life of the pipe, it may be changed by a growth rate change of the crack being caused for some reason. Therefore, to secure safety, it is desired to obtain a capability of monitoring the status of the crack during the operation of the plant. In the above UT or TOFD method, however, a probe cannot be used in a high-temperature environment (500 to 600), and therefore it has been impossible to apply those methods to the monitoring in the above.
In view of this problem, it is an object of the present invention to provide a crack monitoring method and apparatus which is capable of monitoring a crack inside piping precisely and easily at all times even in a high-temperature environment during the operation of a plant.
According to a first aspect of the present invention to solve the above problem, there is provided a crack monitoring method for monitoring a crack inside piping, comprising attaching a pair of heat-resistant current input-output electrodes to an outer circumferential surface of the piping such that the crack whose position is previously specified is put between the pair of heat-resistant current input-output electrodes and attaching at least one pair of heat-resistant crack electric potential difference measuring electrodes to the outer circumferential surface of the piping such that they are put between the current input-output electrodes, supplying an alternating current between the current input-output electrodes and measuring an electric potential difference between the crack electric potential difference measuring electrodes, correcting the electric potential difference between the crack electric potential difference measuring electrodes, and calculating the length of the crack on the basis of the electric potential difference obtained and corrected.
According to a second aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the attached positions of the current input-output electrodes and the crack electric potential difference measuring electrodes are set on the basis of an electric field analysis of the portion of the welded portion of the piping surrounding the crack so that the electric potential difference can be measured most sensitively at a growth of the crack in the piping.
According to a third aspect of the present invention, there is provided a crack monitoring method according to the second aspect of the invention, wherein a pair of crack electric potential difference measuring electrodes are attached to the piping such that the crack on the center side is put between the electrodes in a direction perpendicular to the direction of likely growth of the crack in the piping.
According to a fourth aspect of the present invention, there is provided a crack monitoring method according to the third aspect of the invention, wherein the other pair of crack electric potential difference measuring electrodes are attached to the piping such that the crack on the tip side thereof in the direction of likely growth is put between the electrodes in a direction perpendicular to the direction of likely growth of the crack in the piping.
According to a fifth aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the current input-output electrodes and the crack electric potential difference measuring electrodes are removably attached to an outer circumferential surface of the piping.
According to a sixth aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein an alternating current of a frequency of 10 Hz or less is supplied between the current input-output electrodes.
According to a seventh aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein an alternating current of a frequency of 50 Hz or greater is supplied between the current input-output electrodes.
According to an eighth aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein an alternating current of a frequency of either 10 Hz or less or 50 Hz or higher are supplied alternately between the current input-output electrodes.
According to a ninth aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the intra-crack electric potential difference correction process is used to correct the electric potential difference between the crack electric potential difference measuring electrodes on the basis of the electric potential difference between the pair of heat-resistant correction electric potential difference measuring electrodes attached to the outer circumferential surface of the piping.
According to a tenth aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the intra-crack electric potential difference correction process is used to correct the electric potential difference between the crack electric potential difference measuring electrodes on the basis of the temperature of the piping.
According to an 11th aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the intra-crack electric potential difference correction process is used to correct the electric potential difference between the crack electric potential difference measuring electrodes on the basis of the internal pressure of the piping.
According to a 12th aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the crack length calculation process is used to calculate the length of the crack from the electric potential difference obtained by the intra-crack electric potential difference correction process on the basis of a correlation between the length of the crack obtained from an electric field analysis of a surrounding portion of the welded portion of the piping containing the crack or experimentally and the electric potential difference in the crack.
According to a 13th aspect of the present invention, there is provided a crack monitoring method according to the first aspect of the invention, wherein the crack length calculation process further comprises a first step of obtaining a correlation between the maximum crack length of the crack for each stress change and an electric potential difference change in the crack on the basis of the electric field analysis of the portion of the welded portion of the piping surrounding the crack in the piping or experimentally, a second step of obtaining a stress change between two points in time, that is, the time when a high load is applied and the time when a low load is applied on the piping on the basis of the temperature and the internal pressure of the piping, and a third step of obtaining a length 2a of the crack corresponding to the stress change from the electric potential difference change between the above two points of time.
On the other hand, to solve the above problem, according to a 14th aspect of the present invention, there is provided a crack monitoring apparatus for monitoring a crack generated inside the piping, comprising a pair of current input-output electrodes made of heat-resistant material attached to an outer circumferential surface of the piping, a sensor head having at least one pair of crack electric potential difference measuring electrodes made of heat-resistant material attached to the outer circumferential surface of the piping such that they are put between the current input-output electrodes, an electric alternating current power supply for supplying an alternating current between the current input-output electrodes of the sensor head, a crack electric potential difference measurement means for measuring the electric potential difference between the crack electric potential difference measuring electrodes of the sensor head, an intra-crack electric potential difference correction means for correcting an electric potential difference between the crack electric potential difference measuring electrodes of the sensor head measured by the crack electric potential difference measurement means, and a crack length calculation means for calculating a length of the crack on the basis of the electric potential difference obtained by the intra-crack electric potential difference correction means.
According to a 15th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the positions of the current input-output electrodes and those of the crack electric potential difference measuring electrodes of the sensor head are set on the basis of an electric field analysis of the portion of the welded portion of the piping surrounding the crack in the piping so that the electric potential difference can be measured most sensitively at the growth of the crack in the piping.
According to a 16th aspect of the present invention, there is provided a crack monitoring apparatus according to the 15th aspect of the invention, wherein the pair of crack electric potential difference measuring electrodes of the sensor head are attached to the piping such that the crack on the center side is put between the electrodes in a direction perpendicular to the direction of the likely growth of the crack in the piping.
According to a 17th aspect of the present invention, there is provided a crack monitoring apparatus according to the 16th aspect of the invention, wherein the other pair of crack electric potential difference measuring electrodes of the sensor head are attached to the piping such that the crack on the tip side thereof in the direction of likely growth is put between the electrodes in a direction perpendicular to the direction of likely growth of the crack in the piping.
According to a 18th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein there is provided an attachment means for removably attaching the sensor head to the outer circumferential surface of the piping.
According to a 19th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein an alternating current of a frequency of 10 Hz or less is supplied by the alternating current power supply.
According to a 20th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein an alternating current of a frequency of 50 Hz or greater is supplied by the alternating current power supply.
According to a 21st aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein an alternating current which is alternately either 10 Hz or less or 50 Hz or higher is supplied by the alternating current power supply.
According to a 22nd aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the intra-crack electric potential difference correction means comprises a pair of correction electric potential difference measuring electrodes made of heat-resistant material arranged in the sensor head so as to be attached to the outer circumferential surface of the piping and the correction electric potential difference measurement means for measuring an electric potential difference between the correction electric potential difference measuring electrodes.
According to a 23rd aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the intra-crack electric potential difference correction means comprises a piping temperature measurement means for measuring temperature of the piping.
According to a 24th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the intra-crack electric potential difference correction means comprises a means for measuring the internal pressure of the piping.
According to a 25th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the crack length calculation means calculates the length of the crack from the electric potential difference obtained by the intra-crack electric potential difference correction means on the basis of a correlation between the length of the crack obtained from an electric field analysis of the welded portion of the piping surrounding the crack or experimentally and the electric potential difference in the crack.
According to a 26th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the crack length calculation means performs a first step of obtaining a correlation between the maximum crack length of the crack for each stress change and an electric potential difference change in the crack on the basis of the electric field analysis of the welded portion surrounding the crack in the piping or experimentally, a second step of obtaining a stress change between two points in time, that is, the time when a high load is applied and the time when a low load is applied on the piping on the basis of the temperature and the internal pressure of the piping, and a third step of obtaining the length of the crack corresponding to the stress change from the electric potential difference change between the above two points of time.
According to a 27th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein a plurality of sensor heads are arranged, the intra-crack electric potential difference measurement means measures the electric potential differences between the crack electric potential difference measuring electrodes of each of the plurality of sensor heads, the intra-crack electric potential difference correction means corrects the electric potential differences between the crack electric potential difference measuring electrodes of the plurality of sensor heads measured by the crack electric potential difference measurement means, and the crack length calculation means calculates the lengths of the cracks on the basis of the electric potential differences obtained by the intra-crack electric potential difference correction means.
According to a 28th aspect of the present invention, there is provided a crack monitoring apparatus according to the 14th aspect of the invention, wherein the intra-crack electric potential difference measurement means, the intra-crack electric potential difference correction means, and the crack length calculation means are arranged in a remote area far from the piping.